We propose to study the function of a brain neurochemical, dopamine (DA), in unipolar and bipolar depressed patients and healthy controls, both at baseline and after a pharmacological challenge. The baseline levels will characterize the normal function of the dopaminergic nerve terminals (tonic release). The levels after a pharmacological challenge will characterize the responsiveness of the nerve terminals to stress (phasic release). By obtaining both measures, we would have a complete characterization of DA function. These measurements will be obtained with the technique of single photon computerized tomography (SPECT), based on the principle of competition between a radiotracer, [123I]IBZM, and DA for binding to the same receptor in the brain, the D2 receptor.